An important thing about battle spells is that it can only be cast in a
battle that involves at least 200 people (so the fighting must have
started and the spells can only be cast on units on the battlefield).
That is an awfull lot (in my campaign at least). But I agree that these
spells can be dangerously powerfull as my players found out when a 'mass
sanctuary' was used against them. I do like the idea of certain battle
spells though, it will make priests and mages very valuable in battles
on both sides. The fact that battle spells existed, actually made my
game very enjoyable. Temples get a lot more involved in politics,
because no regent would want to start a war without the backing of a
temple. The regent would need the priests for the dispel magics and
simular spells. Before that my players ignored temples more or less (I
know that some realm spells are rather good). Not to mention the fact
that PC's actualy have a central role in battles: defend their own
spellcasters, while trying to assassinate or capture the mages and
priests of the other side. This last fact is also a good power limit on
battle spells. No sane spellcaster will remember many battle spells,
since the assassins sent out to kill them will not be in large groups.So
the spellcaster needs a lot of personal spells.
Secondly you as a DM have got full control over battle spells and it
would not be unfair to rule certain spells out. Limiting the duration of
any spell to the length of the battle might be a very logical one.
Afterall the spell is fueled by the battle itself.
Thirdly and most importantly: how many spellcasters are there in the
world and how many actualy want to risk their lives on a battlefield?